The present invention relates to sterilisation apparatus.
It is well known that ultraviolet radiation has sterilisation properties. Typically arc lamps are used to produce radiation of an appropriate wavelength. These lamps must be connected to special power supplies and the apparatus is therefore somewhat cumbersome in use. Furthermore, the lamps have a limited life and suffer from shadowing problems caused by their connecting cables and internal electrodes. In the sterilisation field, shadowing can result in parts of an article receiving an insufficient cumulative level of radiation to achieve effective sterilisation.
It has recently been appreciated that an ultraviolet bulb which is energised by microwave radiation may provide a solution to at least some of the problems associated with arc lamps.
U.S. Pat. No. 5,166,528 describes much of the research performed to date. This patent describes a nonconductive housing which supports one or more microwave energisable ultraviolet bulbs and which is adapted to be placed within the microwave cavity of a conventional microwave oven. An article to be sterilised is placed within the cavity either within or adjacent the housing. In all the described embodiments, the article to be sterilised is exposed to microwave radiation. The patent acknowledges that heating of the article by the microwaves occurs but comments that this is limited because sterilisation only takes 30 to 40 seconds, for example. However, in practice, the applicant has found this to be a significant problem, firstly because any heating of the article may be unacceptable as is the case with some contact lenses, for example, where heating causes distortion of the lens. Secondly, the length of sterilisation needed may be more than 40 seconds, and thirdly, heating of the article is not the only consideration; it is not possible, for example, to sterilise a conductive article using the arrangements described in the patent. Thus, the practical applications of the teaching of U.S. Pat. No. 5,166,528 are limited.
It is an object of the present invention to improve on the teaching of U.S. Pat. No. 5,166,528 and in particular, to solve the problem of sterilisation of microwave sensitive articles using at least one microwave energised ultraviolet bulb.
Accordingly, in a first aspect thereof, the invention provides a steriliser comprising an openable enclosure for surrounding one or more articles to be sterilised, the enclosure being arranged to attenuate microwave radiation such that in use, when the enclosure is irradiated with microwave energy, the microwave field energy throughout the interior of the enclosure is significantly less than that outside the enclosure, and the enclosure including a gas tight cavity containing a gaseous charge, the charge being chosen from the group of materials which emit ultraviolet radiation when irradiated with microwave radiation.
Typically the gaseous charge is of mercury or a metal halide and the enclosure is substantially constructed of an ultraviolet transmissive material such as quartz which is sufficiently heat resistant, microwave transparent and ultraviolet transparent for the purposes of the present invention. By arranging for the enclosure to attenuate the microwave radiation whilst still allowing ultraviolet radiation to penetrate, the disadvantages of the prior art are avoided.
The attenuation may be effected using only the gaseous charge which substantially transforms the incident microwave energy from the microwave to the ultraviolet spectrum by absorbing most of the microwave wavelength and retransmitting the energy in the ultraviolet spectrum. This results in an effective attenuation of microwave energy incident on the article in the enclosure. Alternatively, attenuators such as water (which convert the energy to heat) or electrically conductive elements (arranged to adjust the microwave field in accordance with conventional field theory) may be used. The object of the attenuation is to maximise exposure of the article to ultraviolet radiation at germicidal wavelengths (typically around 260 nm) whilst reducing the microwave energy field to a level insufficient to damage the article or in the case of conductive articles, insufficient to allow arcing and consequent damage to the microwave source.
The preferred embodiment uses only the gaseous charge to attenuate the energy level of the microwave radiation within the enclosure since this results in the most efficient use of the microwave energy (since the energy is largely converted to ultraviolet radiation rather than heat).
According to a second aspect of the invention, a method of sterilising an article comprises placing one or more of the articles into an enclosure, the enclosure including a gas tight cavity containing a gaseous charge, the charge being chosen from the group of materials which emit ultraviolet radiation when irradiated with microwave radiation, and irradiating the enclosure with microwave radiation, the enclosure being arranged such that the energy level of microwave radiation inside the enclosure is less than that outside and is insufficient significantly to damage the article and whereby the article is sterilised by irradiation with ultraviolet radiation.